1. Field of the Invention
The present invention relates to a liquid crystal display device including a liquid crystal layer sandwiched between a pair of substrates.
2. Description of the Related Art
Liquid crystal displays displaying images by driving liquid crystal display devices have a low profile, a light weight, and low power consumption, so the liquid crystal displays are widely used for not only image displays such as televisions or monitors but also information terminals such as digital cameras or cellular phones.
Such liquid crystal displays are classified into liquid crystal display systems (display modes) of the liquid crystal display devices, and a twisted nematic (TN) mode is well known. However, there is increasing interest in a vertical alignment (VA) mode, because the VA mode achieves a wider viewing angle than the TN mode.
In the VA mode, for example, liquid crystal molecules in a liquid crystal material has a negative dielectric constant anisotropy, that is, a property of having a smaller dielectric constant in a long axis direction of the molecules than that in a short axis direction thereof, and liquid crystal molecules aligned vertically to a substrate respond so as to be aligned horizontally to the substrate, thereby light is modulated and transmitted therethrough. In the VA mode, the liquid crystal molecules are aligned in any direction, so the alignment directions of the liquid crystal molecules vary, thereby responsivity to voltage declines. Therefore, to improve the responsivity, a rubbing process for controlling the alignment of liquid crystal molecules is performed. However, in a liquid crystal display device subjected to the rubbing process, scratches due to rubbing easily cause display unevenness, and unless alignment division for arranging a plurality of regions with different alignment directions of liquid crystal molecules is performed, it is difficult to secure a wide viewing angle. Therefore, a technique of controlling the alignment of liquid crystal molecules except for the rubbing process has been studied.
As the technique of controlling the alignment of liquid crystal molecules, for example, as illustrated in FIG. 12, a technique of arranging linear projections on a surface of a substrate is known. In a liquid crystal display device using the technique, a liquid crystal layer 500 including liquid crystal molecules 500A is sealed between a drive substrate 200 and a facing substrate 300. Electrodes 202 and 302, linear projections 410 not facing one another and alignment films 400 being laid over the electrodes 202 and 302 and the linear projections 410 are arranged on facing surfaces of the drive substrate 200 and the facing substrate 300. In the liquid crystal layer 500, in a state in which a voltage is not applied, the liquid crystal molecules 500A are aligned substantially vertically to surfaces of the alignment films 400. Therefore, while the liquid crystal molecules 500A in regions in proximity to the linear projections 410 are slightly tilted with respect to the surfaces of the drive substrate 200 and the facing substrate 300 (that is, a tilt angle is provided), the liquid crystal molecules 500A in other regions are aligned substantially vertically to the surfaces of the drive substrate 200 and the facing substrate 300. When a voltage is applied to the liquid crystal layer 500 in this state, the tilt of the liquid crystal molecules 500A in proximity to the linear projections 410 successively propagate to other liquid crystal molecules 500A, and these liquid crystal molecules 500A respond so as to be aligned substantially horizontally to the surfaces of the drive substrate 200 and the facing substrate 300.
However, in the liquid crystal display device illustrated in FIG. 12, there is a lag between a timing when the liquid crystal molecules 500A aligned vertically to the surfaces of the drive substrate 200 and the facing substrate 300 fall down in response to the application of a voltage and a timing when the liquid crystal molecules 500A in proximity to the linear projections 410 fall down in response to the application of a voltage. As a result, there is an issue that the response speed of the liquid crystal molecules 500A as a whole becomes slower. In particular, in a gray-scale change from black to an intermediate color, the change amount of an applied voltage is small, so the response speed becomes further slower.
Therefore, in the VA mode, techniques of slightly tilting liquid crystal molecules from a direction of normal to a substrate by a polymer material and keeping the liquid crystal molecules slightly tilted are known as described in Japanese Unexamined Patent Application Publication Nos. 2002-357830 and 2003-307720. More specifically, after a liquid crystal layer formed by adding a monomer having photopolymerizability is sandwiched between substrates, the liquid crystal layer is exposed to light under the application of a voltage for tilting liquid crystal molecules, to polymerize the monomer so as to form a polymer, thereby a pretilt angle is provided to the liquid crystal molecules, and a tilting direction of the liquid crystal molecules under no application of the voltage is predetermined.